1. Field of the Invention
The present invention relates to a full-color type light emitting diode display unit (hereinafter referred to simply as an "LED unit"), and more particularly to an LED display unit which is expanded in a light distribution angle, and is increased in luminance while improving quality of mixed colors and white balance.
2. Description of the Related Art
As a result of successful production of high luminance blue LEDs, it becomes possible to manufacture a full-color display unit wherein three primary colors of blue, red, and green are used. A large-sized display wherein such LED display units are used has been manufactured, and such a large-sized display is fixed to, for example, the wall of a building or the like, so that it attracts walker's eyes. Meanwhile, high luminance of green LEDs has been also realized recently, whereby full-color display units became possible to be fabricated by the use of the same number of LEDs as those of red and blud LEDs. In this connection, a larger number of green LEDs have been heretofore used than those of red and blue LEDs.
LED display unit is classified generally into the following two types.
(1) Large-sized display
One dot is formed by utilizing a plurality of cannon ball-shaped LED lamps each having different luminescent colors, one module to display one Chinese character is composed by horizontally and vertically arranging 16.times.16 lamps (=256 dots), and a plurality of the modules are horizontally and vertically arranged. The resulting large-sized display is principally employed for display of information on the wall of a building or along an express-highway.
(2) Small- to medium-sized display
One dot of a cannon ball-shaped LED lamp is formed by employing a plurality of LED chips each having different luminescent color, one module is composed by horizontally and vertically arranging 16.times.16 lamps (=256 dots), and a plurality of the modules are horizontally and vertically arranged. The small- to medium-sized display is principally employed for display of information on schedules in a train station yard.
In such a full-color type LED display unit as described above, a plurality of LED chips or a plurality of LED lamps each having different luminescent color are used for pixels, so that the following characteristics are required.
(1) Excellent quality of mixed colors
(2) Excellent white balance
(3) Wide light distribution angle
(4) High luminance
FIGS. 1A and 1B show a cannon ball-shaped LED wherein a plurality of the LED chips are used. The LED lamp 4 is one employed principally for a small- to medium-sized display, and made from a resin which is added with a light diffusing material. The LED lamp 4 comprises therefor a lens 40 for sealing three LED chips 20G (green), 20R (red), and 20B (blue); and leads 41 for connecting the LED chips 20G, 20R, and 20B to wiring patterns of a printed-circuit board.
FIG. 2 shows an LED display unit in which the LED lamps 4 are used to be arranged in matrix. The LED display unit is constituted by horizontally and vertically arranging 16.times.16 LED lamps 4 (=256 dots) on a printed-circuit board 10. In this LED display unit, for preventing short circuit of wiring patterns on the printed-circuit board caused by close arrangement of leads of the LED lamps 4 on the printed-circuit board, the LED chips 20G, 20R, and 20B are obliquely arranged to widen the interval of the leads 41 of the LED lamp 4.
FIG. 3 is a graphical representation showing results obtained by measuring light distribution angle of the LED display unit in the A--A direction in FIG. 2, wherein the abscissa indicates the light distribution angle, and the ordinate indicates relative luminance based on the maximum luminance of 1. As is apparent from the figure, all the three colors have a relative luminance of 1/2 or more in the "a", and at least one of the colors has a relative liminance of 1/2 or more in the regions "b1" and "b2". On the other hand, all the three colors have a relative luminance of 1/10 or more in the region "c". From the above results, the contents displayed on the LED display unit can be viewed in the range "c" of approximately 90.degree..
In the conventional LED display unit, howevers, there arises color collapse, and particularly, luminance of green is low in the region "b1", while it is high in the region "b2", so that sufficient white balance cannot be obtained, since three colors of LED chips have been obliquely arranged. Furthermore, although red color is hard to mix with either blue color or green color, the relative luminance of both blue and green is positioned on the upper and lower sides of the relative luminance of red color in the regions "b1" and "b2", so that good quality of mixed colors cannot be obtained.